1) the resulting reflected light is near worthless because the light above only penetrates too far. trim up your plants to this point, adjust lights above accordingly.
2) paint is likely toxic
3) foil would result in a big mess
If you want to provide more light then put another light above if you can, otherwise consider
alternative training to maximize the light you have. Like a scrog, short and squat young plant(s) flowering in an even canopy
can outperform many large plants I've seen. Train and top in veg. I once used a mini laundry basket in a locker cabinet. I cut a hole in the bottom and as she grew up inside the basket I trimmed and weaved every new meristem into a new hole, in and out to fill all of them. when they were full I flowered under a 250 watt hps and it was awesome. good luck